While diesel and lean-burn gasoline engines are known to provide beneficial fuel economy, these engines also produce oxides of nitrogen (NOx) and particulates during normal operation. Several methods are known for aftertreatment of the emission stream to reduce NOx to acceptable regulatory levels.
One such method utilizes a lean NOx trap (LNT), which typically includes a NOx adsorber combined with a catalyst for NOx reduction. The LNT operates in a cycle of lean and rich exhaust conditions, i.e., the LNT stores NOx under lean (i.e., excess oxygen) exhaust conditions followed by NOx reduction over the catalyst when the engine is running under rich conditions. The reduction occurring during the rich cycle purges the LNT so that it is regenerated for the next storage cycle. The LNT typically utilizes a platinum group metal catalyst (PGM) to aid in both NOx storage and reduction. However, high concentrations of platinum metal groups are required in LNTs in order to achieve high NOx conversion levels, and such metals are relatively expensive. In addition, the reaction temperature window can be limited, and the fuel penalty associated with the rich purges can cut significantly into the fuel economy benefits of lean operation. Another problem with the use of lean NOx traps is the generation of ammonia by the trap, which may be emitted into the atmosphere during rich pulses of the LNT.
Another exhaust aftertreatment system in use utilizes a selective catalytic reduction (SCR) catalyst, which selectively reduces NOx in the presence of excess oxygen. An SCR catalyst provides NOx conversion at minimal cost compared with the use of PGM catalysts as SCR catalysts are based on transition metal oxides. However, selective catalyst reduction requires the presence of a suitable reductant species in the exhaust that reacts with NOx rather than the excess oxygen. The reductant may comprise hydrocarbons which result from the incomplete or partial oxidation of fuel. However, in diesel applications, there are often insufficient hydrocarbons present to achieve conversion of NOx. In this instance, reducing agents such as urea or ammonia are carried on-board the vehicle and injected into the exhaust stream, typically upstream from the SCR catalyst. Urea-SCR technology eliminates the need for the rich purges and can potentially offer a broader operating temperature window. However, a disadvantage of liquid reductants is that freezing of the reductant may occur. Liquid reductant freezing is particularly a problem for the use of diesel vehicles in cold-weather climates when a liquid reductant is employed.
Another known exhaust aftertreatment system utilizes an LNT in combination with an NH3-SCR catalyst. The NH3-SCR catalyst is positioned downstream from the LNT and provides additional NOx conversion. In this configuration, the selective catalytic reduction of NOx over the SCR catalyst typically occurs via ammonia (NH3) generated over the upstream LNT and not via reductant injected into the exhaust. See, for example, commonly-assigned U.S. Publication No. US 2007/0144153. The mechanism for such a system is based on ammonia forming at the LNT under rich conditions, which then slips out and is adsorbed on the downstream SCR catalyst. During the lean portion of the lean-rich cycle, NOx passing through the LNT is then reduced at the SCR catalyst using the stored NH3 as a reductant. However, one drawback of this system is the need for excessively rich LNT purge conditions in order to generate NH3 breakthrough (slip) from the LNT, which results in a fuel economy penalty. In addition, NH3 may slip through the downstream SCR catalyst and be emitted into the atmosphere or may react on the SCR catalyst to re-form NOx under high temperature conditions.
Accordingly, there is a need for an improved exhaust aftertreatment system which has high NOx conversion over a broad range of temperatures, which does not incur significant fuel penalties, and does not suffer from the drawbacks of the use of an ammonia or urea reductant.